If your aim is to land a contract with one of the major book publishing houses, you probably will need an agent to represent your work. About 80 percent of the books these conglomerates publish are purchased through agents. Some of the largest houses won't even consider submissions from unrepresented writers; when they get manuscripts directly from the author, the author usually gets a short form note advising him to get an agent.
The advantage to the big publishers in dealing only with agents is that agents know what editors are looking for and won't submit work that isn't salable. The agent's reputation, and therefore his ability to succeed as a agent, rides on submitting only the best--not just in terms of ideas, but also in terms of presentation and research--to only those editors who are appropriate for the project. The publisher saves enormous time and expense by allowing agents to do the work of shifting through submissions to find the real gems.
Meg Schneider and Barbara Doyen